Multiple calcrete profiles in the Tabernas Basin, southeast Spain: their origins and geomorphic implications

Abstract

This paper describes a sequence of Pliocene(?) to Quaternary age calcretes developed within alluvial fan and fluvial gravels in the Tabernas Basin, Almería Province, southeast Spain. Calcrete profiles are described from sites adjacent to major tributaries of the Rambla de Tabernas. Six distinct calcrete units are identified within the basin. These have variable distributions but have developed in an identifiable evolutionary sequence. Two pairs of calcrete units are widely present across the basin preserving two former land surfaces. Each of the former land surfaces has been planated and subsequently buried by alluvial fan or fluvial gravels. A massive calcrete unit is present at the base of each gravel sequence, immediately in contact with the underlying bedrock, with a less well developed calcrete unit situated at the top of the gravel sequence. The lowest two calcrete units within the basin are more spatially restricted and are confined to the floors and flanks of incised drainage lines.The geochemistry, macro-and micromorphological properties and geomorphological positions of the calcrete units are outlined and, on the basis of this information, their mode of origin identified. Two main modes of calcrete genesis appear to be present: massive calcretes situated in direct contact with bedrock are suggested to have formed by groundwater processes, whilst calcretes situated at the top of gravel sequences are likely to have developed by pedogenic processes. Calcrete genesis is subsequently considered in the context of the reconstruction of the early phases of landscape development, and is suggested to have been controlled by phases of uplift and stability within the Tabernas Basin.

Nature of sedimentary deposits in the western Makgadikgadi basin, Botswana

“…The ground-water (nonpedogenic) calcretes are developed when carbonate is precipitated in the capillary fringe of a fluctuating water-table (Mann & Horwitz, 1979;Arakel, 1986;Nash, 1997;Watson & Nash, 1997). This involves the lateral transfer of carbonate rich solutions through the host sediment (Nash & Smith, 1998). Pedogenic calcretes develop when carbonate is precipitated vertically within the soil profile after being introduced mainly from external atmospheric or biogenic sources (Goudie, 1983;Wright & Tucker, 1991).…”

The West Water Formation (Hualapai Plateau, Arizona, USA) as a calcrete-paleosol sequence, and its implications for the Paleogene-Neogene evolution of the southwestern Colorado Plateau

“…Overprinting of groundwater calcrete by pedogenic calcrete is not uncommon, particularly where the former has become exhumed as a result of surface erosion (cf. Khadkikar et al, 1998;Nash and Smith, 1998;Alonso-Zarza, 2003). Pedogenic calcrete emplacement may also explain the apparent decrease in the relative proportion of detrital grains present towards the very top of the massive calcareous unit, as expansion and grain displacement is widely documented in association with near-surface carbonate crystal growth (e.g.…”

“…This may lead to mixed alpha and biogenic (beta) fabrics in the uppermost sections of the resulting calcrete profile, with features such as microbial coatings, calcified tubules, calcified pellets and plant cells, spherulites, and alveolar septal structures present in addition to the alpha-fabric features noted above (cf. Nash and Smith, 1998;Nash and McLaren, 2003;Wright, 2007).…”

“…Many cracks and void spaces between nodular features are filled by microspar, with sparite present in some voids towards the base of the exposures. The presence of more sparry cements within otherwise simple micritedominated fabrics is documented in a variety of groundwater calcretes (Tandon and Narayan, 1981;Nash and Smith, 1998;Mack et al, 2000). Similar vertical variability in calcite crystallinity has been described in channel calcretes in southeast Spain by Nash and Smith (2003), where it is suggested to be related to longer porewater residence times in lower sections of profiles.…”

Coupled micromorphological and stable isotope analysis of Quaternary calcrete development

“…This study has focused 502 on pedogenic calcretes, but groundwater carbonates are also widespread in arid and semi-arid regions, 503 including southeast Spain (e.g. Nash and Smith, 1998). This approach may provide opportunities to 504 explore in detail the relationships between groundwater calcretes and palaeoenvironmental conditions.…”